(1) Field of the Invention
The present invention relates to a resin useful in preparing plastic lenses having a high-refractive index, lenses containing the resin, and a method for the preparation of such lenses.
(2) Description of the Prior Art
In recent years, plastic lenses are rapidly being more frequently utilized as optical elements in eyeglasses, cameras and the like, because they are lighter, more unbreakable and more dyeable than inorganic lenses.
A resin which can be prepared by the radical polymerization of diethylene glycol bis(allyl carbonate) (hereinafter referred to simply as DAC) has been used to prepare such lenses. This resin has a high impact resistance, is lightweight, and exhibits excellent dyeing properties and good workability of cutting and polishing.
However, lenses prepared from the above-mentioned resin have a lower refractive index (n.sub.D =1.50) than inorganic lenses (n.sub.D =1.52). In order to obtain equivalent optical properties to glass lenses, it is necessary to increase the central thickness, peripheral thickness and curvature of the lens and hence the lens becomes very thick. For this reason, resins having a high-refractive index are desired. Resins which have been previously used to prepare lenses having a high-refractive index include resins prepared by reacting an isocyanate compound with diethylene glycol (U.S. Pat. No. 4,443,588), reacting an isocyanate compound with a hydroxyl compound containing a halogen atom such as tetrabromobisphenol A (Japanese Patent Laid-open Publication No. 164615/1983), or reacting an isocyanate compound with a hydroxyl compound containing a diphenyl sulfide skeleton (Japanese Patent Laid-open Publication No. 194401/1985).
The present inventors have previously proposed high-refractive plastic lenses comprising polyurethane resins or the like which can be prepared by reacting an isocyanate compound with a hydroxyl compound having a sulfur atom (U.S. Pat. Nos. 4,680,369 and 4,780,552) or reacting an isocyanate compound with a polythiol (U.S. Pat. No. 4,689,387 and Japanese Pat. Laid-open Publication No. 267316/1987).
Lenses comprising previously known resins have a higher-refractive index than the lenses containing DAC, however, the refractive index thereof is still insufficient. Such resins have drawbacks such as high dispersion, poor weatherability and great specific gravity, since the resins are made from compounds containing numerous halogen atoms or aromatic rings to improve the refractive index.
It has been known to add an internal releasing agent during the process for preparing such lenses to improve mold release characteristics. In such process butyl phosphate is added to DAC. However, the addition of butyl phosphate is not required, and results in impaired appearance of the molded articles. [Seiichi Mima, "Polymer Digest", 3, 39 (1984) and the like].
In molding polyurethane and S-alkyl thiocarbamic acid ester lenses, it is difficult to release the lens from the mold after polymerization. In order to solve this problem, the present inventors invented a method in which an external releasing agent is employed (Japanese Pat. Laid-open Publication No. 267316/1987) and a method in which a mold made from polyolefin resin is employed (Japanese Pat. Laid-open Publication No. 236818/1987).
However, these above mold release methods are still insufficient for improving mold release property in the cast polymerization because of the lens according to the present invention.
In the method in which an external releasing agent is employed, a surface treatment material for treating the inner surface of the mold is partially transferred to the surface and interior of the polymerized lenses, causing the lens surface to be uneven and turbid. The mold release treatment is required each time the mold is used. Accordingly, this method is intricate, decreases lens productivity, and thus is uneconomical.
Further, a polyolefin resin mold, is deformed at elevated temperatures, resulting in poor accuracy of the lens surfaces. Thus, the method cannot be applied in applications where high surface accuracy is required.